DS 7-76 Prevention and Mitigation of Combustible Dust ... - FM Global

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7-76 Combustible Dust Explosion Page 30 FM Global Property Loss Prevention Data Sheets Where L f,max = maximum flame jet length, m Step 3: For distances R larger than Rmax, calculate the distance-specific blast pressure by the following equation: P blast (barg) = (R max/R) 1.5 P blast,max Note 1: These calculated pressures are very localized near the fireball ejected from a vented explosion. This is not the same as the overall explosion pressure (P red) created throughout the enclosing room or building. In a large building, this shock pressure might be the largest pressure that the walls located close to exploding equipment will feel, but, usually, the pressure on the walls of a room enclosing the vented equipment depends on the quantity of combustion gases produced by the explosion and vented into the room. Such pressure is essentially uniform throughout the room, regardless of distance of the walls from the vented vessel. Note 2: The equations for predicting pressure effects are valid only for a single explosion vent. For multiple vents that open in the same direction, calculate P blast,max for each vent, then add the values to estimate the combined, conservative pressure effect. 3.1.10.2 Estimate the maximum flame jet length in a direction normal to the vent using the following equation: L f,max = 8 V (1/3) where L f,max = maximum flame jet length, m V = Volume of vented enclosure, m 3 This equation is valid only for ST1 dusts (i.e., K st ≤ 200) with P red ≤ 1 barg (14.5 psig) and P stat ≤ 0.1 barg (1.5 psig). There are no published correlations for situations outside the stated limits. It is believed that changes in P stat will change the size of the predicted fireball; however, current estimation methods have not quantified that effect. 3.1.10.3 Estimate the recoil force from an explosion using the following equations (2.3.3.1.18): a) For dynamic recoil force, use the following formula: F R = 119 A v P red Units: F R (kN), A v (m 2 ), P red (barg) F R = 1.2 A v P red Units: F R (lb), A v (in 2 ), P red (psig) b) For duration of this recoil force, use the following formula (applies only to enclosures without vent ducts): t f = 10 -4 K st V/(P red A v) Units: K st (bar m/s), V (m 3 ), P red (barg), A v (m 2 ), 10 -4 (s 2 m -2 ) a constant c) As an alternative to dynamic force anchoring design, use an equivalent static force (F s) as calculated using the following formula: F s = 0.52 F R Units: kN or lb as in the original equation 3.1.11 Fixed Obstructions near the Face of Explosion Vents (2.3.3.1.12) A fixed obstruction located too close to an equipment explosion vent opening creates significant resistance to the free outflow of combustion products from the vented equipment. This is believed to be mainly due to dust burning after being ejected from the protected equipment. Since the combustion occurs in the semiconfined area between the vessel and the obstruction, a condition similar to a secondary explosion occurs. This has significant backpressure effects on the equipment explosion. The state of knowledge regarding this phenomenon is limited, and it is impossible to quantify the effect. It is also not possible to provide any guidance on safe distances to nonflat obstructions, due to the complexity of the effects of different geometries producing partial confinement. The only safe course of action is to locate and orient explosion vents so they do not point to nearby surfaces. ©2008 Factory Mutual Insurance Company. All rights reserved.
Combustible Dust Explosion 7-76 FM Global Property Loss Prevention Data Sheets Page 31 3.1.12 Distribution of Explosion Vents (2.3.3.1.15) If a protected vessel has significant obstructions within it, there are two principles of explosion that make it important to provide well-distributed explosion venting. First, if the gases heading toward an explosion vent at one end of the vessel have to flow through obstructed areas, the gases will not flow out as fast as they would if the volume was unobstructed. Because of this, the vented explosion pressure P red can rise above the expected level. Second, if the gases heading toward the explosion vent flow over any significant obstructions, turbulence will increase substantially within the vessel. Because the rate of pressure rise of an explosion increases with increasing turbulence, the obstacles can worsen the explosion. Vents distributed throughout an entire enclosure will help ensure the gases vented during the explosion take the shortest path out of the enclosure. 3.1.13 Dust Collector Operating Pressures (2.3.3.1.17) The normal operating pressure range for dust-collecting and dust-handling systems is +40 to 40 mbar (+16 in. H 2O to 16 in. H 2O). In this pressure range, no adjustments to the standard explosion venting guidelines are necessary. 3.1.14 Explosion Isolation (2.3.3.2) When an explosion occurs in an enclosure protected by pressure containment, both pressure and flame (i.e., burning dust) propagate down any open, connected ductwork to another enclosure. If that second vessel also has a pressure-resisting design, it is likely to be insufficient due to an effect often referred to as ‘‘pressure piling.’’ The pressure in the second vessel will increase before the source of ignition (i.e., burning material) arrives, as the pressure disturbance travels from the first vessel to the second at the speed of sound, which is generally greater than the speed of the flame front. Therefore, at the moment when a dust explosion is ignited within the second vessel, the initial pressure will be well above the normal (ambient) pressure. For a given fuel-to-air ratio, the final, unvented pressure of an explosion is directly proportional to the initial pressure. For example, if the first explosion prepressurizes the second vessel to 3 bar (44 psi) absolute, then the final pressure of the explosion in the second vessel would increase by a factor of 3. For a dust with a P max value of 9 bara, the final unvented pressure in this example would be 27 bara, well above the strength of even the most sturdy vessel designed for dust explosion pressure containment. Thus, where explosion containment is used as a protection method, it is important to provide explosion isolation to prevent pre-pressurizing a vessel by another explosion. When one vessel protected by explosion containment is connected to a second vessel protected with explosion venting, a protection problem exists that is not related to pre-pressurization, but rather to the turbulence created by the pressure front and a very strong ignition source from the flame front. The result is a more rapid explosion in the second vessel, unaccounted for in the vent design, with failure of the second vessel likely. These effects occur to a lesser extent if the connected vessels are both protected with explosion venting. Thus, where explosion venting is used as a protection method, it is important to provide explosion isolation to separate the vented vessel from any connected vessels protected by explosion containment. 3.1.15 Explosion Isolation: Use of Active Devices (2.3.3.3.4) When using active (as opposed to passive) devices for explosion isolation, the maximum speed the explosion flame front travels from the point of detection to the isolation device is an important consideration. An equation has been proposed correlating the speed of flame propagation in a duct as a function of the pressure developed in the duct. (The duct is filled with an initially quiescent mixture.) ( ) Va = p – po po ρc p 1/γ where: V a is the speed of flame propagation (m/s) p is the maximum pressure in the duct, absolute (Pa) (1 psia = 6897 Pa) ©2008 Factory Mutual Insurance Company. All rights reserved.
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DS 7-76 Prevention and Mitigation of Combustible Dust ... - FM Global

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